1. Field of the Invention
The present invention relates to a structure of a wire bonder for connecting electrodes of a semiconductor chip mounted on a package to external leads with fine metal wires.
2. Description of the Prior Art
The connection of a transistor or integrated circuit chip mounted on a package or a lead frame, to the external leads of the package or lead frame are conducted by the steps of: bringing a hydrogen flame close to the tip of a fine metal wire which runs through a capillary, so as to melt the tip of the fine metal wire to prepare a ball-shaped tip; pressing the ball-shaped tip of the metal wire onto an electrode of the chip after the hydrogen flame has been returned to its original position; subsequently moving the capillary to a position above the external lead and bonding the metal wire to the external lead by means of thermocompression bonding technique; and then cutting the metal wire. These steps are conducted automatically so as to increase the production efficiency and speed. If the wire bonding operation is speeded up, the speed of movement of the hydrogen flame must be increased accordingly and the high-speed movement makes the hydrogen flame unstable. Therefore, a spark discharge is used in recent years to melt the tip of the metal wire in place of the hydrogen flame.
Gold of a high purity is conventionally used as the fine metal wire, but since a fine wire of high-purity gold is mechanically weak and is often broken by the high-speed movement of the capillary, a fine wire of gold containing some impurities has been devised to increase its strength for the automatic wire bonder. Because the price of gold has risen, however, the use of a less expensive, fine silver wire is being examined. If this kind of metal wire is molten in air, oxides are grown on the surface of the molten metal, which raise problems such as a reduction in the bonding strength, or an increase in the electrical resistance of the bond portion. Depending upon the quantity of oxides, moreover, the shape of the molten ball may be distorted or its surface roughened to reduce the bonding strength. In order to prevent the oxidation, or limit it to a suitable rate, therefore, the atmosphere in which the tip of the fine metal wire is molten has to be selected. Since this atmosphere should either be inert or contain a low concentration of oxygen, an idea has been conceived of installing the entire wire bonder within a predetermined atmosphere. However, this would make maintenance and inspection of the wire bonder difficult, reducing the workability.